1. Field of the Invention
The present invention relates to improvements in a position control method and apparatus using a table driven by a rotative driving device such as a motor.
2. Description of the Related Art
FIG. 5 shows a conventional position control apparatus for a table driven by a stepping motor.
Reference numeral 31 designates a table slidably moved on a base 32 via a guide section extending along an axial direction of the motor. The table 31 and the base 32 move relative to each other along an X direction in FIG. 5. The table 31 has a nut 33 fixed thereto such that it is moved together with the table 31. Fitted through the nut 33 is a threaded shaft 35 which is rotated by a stepping motor 34. The nut 33 has a threaded structure formed in an inner peripheral portion thereof, whereas the threaded shaft 35 has a threaded structure formed in an outer peripheral portion thereof. The nut 33 and the threaded shaft 35 are thus fitted together. Accordingly, the table 31 with the nut 33 fixed thereto is moved along an X axis in a direction according to the rotating direction of the threaded shaft 35. One end of the threaded shaft 35 is coupled to a rotating shaft of the stepping motor 34 by a coupling 36. A bearing 37 is fixed to the base 32 at a location intermediate between the coupling 36 and a driving part of the threaded shaft 35 fitted in the nut 33. The bearing 37 supports the threaded shaft so as to prevent the shaft from shaking in a direction orthogonal to the X direction. Thus, the position of the table is regulated in the direction orthogonal to the X axis. An index scale, not shown, provided on the base 32 is arranged opposite a main scale, not shown, provided on the table 31. A floodlight and a light receiving sensor are provided opposite the main scale via the index scale. Light emitted by the floodlight is transmitted through the index scale and then reflected by the main scale. The light is transmitted through the index scale again and then enters the light receiving sensor. By analyzing a photoelectric conversion signal from the light receiving sensor, the amount of displacement of the table can be accurately detected with high resolution.
The above described positioning apparatus for the table can surely achieve position control that meets an accuracy of the order of 0.1 xcexcm. However, it is difficult to control positioning of the table with an increased accuracy, for example, control the positioning of the table with an accuracy of the order of 10 nm. It has been ascertained that even if the table can be stopped at a desired position, even a disturbance caused by minute vibration or the like may result in a positional deviation of 10 nm or more.
Possible causes of this phenomenon will be described below.
The apparatus that controls positioning of a table constructed as above often uses a ball screw. As shown in FIG. 5, one end of the ball screw is coupled to the stepping motor, whereas the other end is fitted in the nut integrally joined to the table. If the stepping motor is rotated through a very small angle, a response characteristic as shown in FIG. 6 is observed between a rotating force F acting upon the ball screw and rotational displacement D of the ball screw.
In FIG. 6, a point O indicates a state where both the stepping motor and the ball screw are stopped. When the stepping motor is driven in this state, the relationship between the force F and the rotational displacement D moves from the point O toward a point A along a curved characteristic line. This is because if the motor is rotated through a very small angle, then during an initial phase of rotation, the ball screw is finely twisted and thus elastically deformed, so that the rotating force is absorbed by the elastic deformation. In this state, the stepping motor having been rotated through a very small angle, one end of the ball screw coupled to the stepping motor is also rotated through a very small angle, but the part of the ball screw which is fitted in the nut is not rotated. That is, the rotational displacement D before the point A is caused by the torsional elastic deformation of the ball screw, and at this time the ball screw itself has not yet started rolling (rotation relative to the nut). When the motor is further rotated so that the force F reaches the point A, a resistance force against the elastic deformation of the ball screw increases above a rolling frictional force F0 of the ball screw so that the rotating force F amounts to the frictional force F0. When the motor is further rotated in this state, the ball screw starts to slide on the surface of the nut without being further elastically deformed, and thus the ball screw starts to roll. At this time, the ball screw rolls while remaining elastically deformed.
Since the ball screw has started rolling, the motor is further rotated until a point B is reached. Once the point B has been reached, the rotation of the motor is stopped. Then, in the ball screw, which has been elastically deformed, a force is generated which acts to restore the ball screw from the deformation. Even if holding current then flows to the rotor of the stepping motor, it is difficult for this current to completely resist the restoring force. Thus, the restoring force slightly rotates the rotor of the stepping motor. The rotation of the rotor causes the ball screw to be restored from the elastic deformation. The restoration gives the ball screw a tolerance for elastic deformation, that is, allows the ball screw to be elastically deformed again by the amount of restoration. If the table is subjected to a disturbance such as vibration when the ball screw has a tolerance for elastic deformation, then elastic deformation occurs within the range of tolerance so that the table is moved by the amount of deformation. If the motor is stopped before the point A is reached, the ball screw has not rolled yet but has only been elastically deformed. Thus, after the stoppage of the motor, a force to restore the ball screw from its elastic deformation is exerted. Then, the restoration gives the ball screw a tolerance for elastic deformation. This tolerance amounts to 10 nm or more. Therefore, it is difficult to control the positioning of the table with the accuracy of the order of 10 nm.
That is, if an attempt is made to control the positioning of the table with the accuracy of the order of 10 nm using the stepping motor as a driving source, the problem of hysteresis between the rotating force F acting upon the ball screw and the rotational displacement D of the ball screw is encountered.
Thus, this hysteresis, i.e. a nonlinear response must be eliminated, and screws for this purpose have been proposed. One of them is a hydrostatic screw. In this screw, a nut has a recess formed in a threaded surface thereof, and oil is filled between the nut and the screw body. With this screw, the friction between the nut and the screw body is generated only by oil viscosity resistance, and hence the frictional resistance is small, and the screw can be constructed to have high rigidity in a feed direction (thrust direction), while having no rigidity in a direction (radial direction) perpendicular to the feed direction. This prevents vertical vibration caused by bending or waviness of the screw. This hydrostatic screw may be used for all mechanical elements of the feed system. In addition to the hydrostatic screw, an aerostatic nut and the like have been proposed. However, these screws have special constructions and thus require much time and labor to fabricate, leading to high costs.
Further, an apparatus has been proposed, which has a stepping motor, a ball screw, and a piezo actuator which are coaxially arranged, as described in Japanese Laid-Open Patent Publication (Kokai) No. 10-58267. In this apparatus, the stepping motor is used to rotate the screw to drive a nut, which is fitted on the screw, in the axial direction, thereby achieving coarse adjustment driving. Further, the piezo actuator is contracted to achieve fine adjustment driving. However, naturally, this apparatus requires the arrangement of the piezo actuator as well as the stepping motor and ball screw.
It is an object of the present invention to provide a position control apparatus and method for a table member which is capable of controlling the positioning of the table through very small displacement thereof, to thereby prevent the position of the table from deviating even when a disturbance such as vibration is applied to the table.
To attain the above object, in a first aspect of the present invention, there is provided a position control apparatus comprising a base member, a table member movable relative to the base member in a predetermined direction, a threaded shaft rotatably mounted on the base member and elastically deformable when twisted, a motor coupled to one end of the threaded shaft, for rotating the threaded shaft to displace the table member, and a pressurizing and fixing member arranged in a vicinity of a location where the motor is coupled to the threaded shaft, for pressurizing the threaded shaft to be fixed in position.
To attain the above object, in a second aspect of the present invention, there is provided a position control apparatus comprising a base member, a table member movable relative to the base member in a predetermined direction, a threaded shaft rotatably mounted on the base member and elastically deformable when twisted, a motor having a rotating shaft coupled to one end of the threaded shaft, for rotating the threaded shaft to displace the table member, and pressurizing and fixing member for pressurizing the rotating shaft of the threaded shaft to be fixed in position.
In a preferred form of the second aspect, the motor and the pressurizing and fixing means are formed by a vibration wave motor.
In a further preferred form of the second aspect, the vibration wave motor comprises a moving member and a vibrating member which are in frictional contact with each other, and the rotating shaft of the motor is pressurized to be fixed in position through static friction between the moving member and the vibrating member.
To attain the above object, in a third aspect of the present invention, there is provided a position control method for controlling positioning of a table member of a position control apparatus comprising a base member, a table member movable relative to the base member in a predetermined direction, a threaded shaft rotatably mounted on the base member and elastically deformable when twisted, and a motor having a rotating shaft coupled to one end of the threaded shaft, for rotating the threaded shaft to displace the table member, the method comprising the steps of rotating the motor to elastically deform the threaded shaft by twisting the threaded shaft, rotating the motor to rotate the threaded shaft to displace the table member while keeping the threaded shaft elastically deformed, and stopping the motor to keep the threaded shaft elastically deformed.
In a preferred form of the third aspect, in the step of stopping the motor to keep the threaded shaft elastically deformed, one of the threaded shaft and the rotating shaft of the motor is pressurized to be fixed in position.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.